The ionization energies of the neutral rare earths have been derived by means of interpolated values for the energy differences between the 4ƒN6s2, 4ƒN6s7s, and 4ƒN6s8s configurations and the difference in the effective quantum numbers of the 4ƒN6s7s and 4ƒN6s8s configurations. In Ce I the 4ƒ5d2ns serieswas used. The results in eV are: The uncertainty is estimated to be ±0.02 eV in all cases but Ce I for which the estimated uncertainty is ±0.06 eV. Values for La I and Gd I obtained spectroscopically by other workers are also tabulated. The present results are compared with those obtained by means of surface ionization. The calculated result for Tb I was inconclusive because of the lack of sufficient knowledge of the energy-level structure.
JOSEPH READER and JACK SUGAR, "Ionization Energies of the Neutral Rare Earths," J. Opt. Soc. Am. 56, 1189-1194 (1966)